Stress concentration is a phenomenon that can be caused by a hole, sharp bend, or defect in an object when force is applied. This is due to the fact that irregularities in the structure of an object will tend to result in a build up of stress in certain areas. Those locations are typically referred to as stress concentrations, since they represent areas where stress forces are increased. An object that is fairly uniform in structure will tend to be much more resilient than a similar object that has one or more stress concentrations. This is due to the fact that the increased pressure found in a stress concentration typically leaves an object much more vulnerable to damage from snapping off, cracking, or breaking in some other way.
When the structure of an object is fairly uniform, stress will tend to be distributed evenly throughout the substance. One way to visualize this is a flat board with evenly spaced force lines running along the surface. If a hole is drilled into the board, any stress applied to the board will be concentrated on either side of the hole. This is due to the fact that the force lines cannot pass through the material that was removed when the hole was drilled. The force lines must go around the hole, resulting in areas of concentrated stress, making the board more likely to break at that spot, if excess force is applied.
Virtually any object that lacks an entirely uniform structure will contain some type of stress concentration. If the substance that an object is made from contains any contaminants or defects, no matter how small, they will tend to result in the concentration of force lines. That makes it especially important to remove any contaminants when constructing objects that will be subjected to a lot of stress. Small cracks also result in stress concentration, which can lead to much larger fractures over time.
The geometric structure of an object can also result in stress concentration. Some common examples include hollow and right angle beams, and shafts with circumferential grooves or shoulder fillets. Each of these shapes has a different pattern of stress concentration, which can result in the failure of an object or component under certain circumstances. Due to the lack of any supporting material in the center, a hollow square beam will tend to fail if it is twisted. Other objects, such as shafts that have grooves or fillets, can fail under torsion, bending, or axial forces.